JDK 9 RFR of JDK-6226715: (ann) java.lang.annotation.AnnotationTypeMismatchException could not be serialized

[joe darcy]

Hello,

Please review the changes to address

     JDK-6226715: (ann) 
java.lang.annotation.AnnotationTypeMismatchException could not be serialized
     http://cr.openjdk.java.net/~darcy/6226715.0/

Patch below.

The analysis of why is patch is valid requires a bit of explanation.

Like all other exceptions, AnnotationTypeMismatchException is 
serializable. However, its state contains a non-serializable element 
field of type Method. Therefore, previously an 
AnnotationTypeMismatchException with a non-null element field threw an 
exception when serialized. Therefore, the only persisted 
AnnotationTypeMismatchException objects have null element fields in the 
serial output.

The patch changes element to be a transient field, removing it from the 
serial output, *without* changing the serialVersionUID. This would not 
usually be a valid transformation, but limits on what can be serialized 
make it acceptable in this case.

If an old serialized form of an AnnotationTypeMismatchException object 
with a null value for element is deserialized after the patch, the now 
extraneous element information is ignored, yielding a semantically 
correct object (with a null value for element).

If a new serialized form is deserialized against the previous version of 
AnnotationTypeMismatchException, the information for the element field 
is missing, but it gets defaulted to be null, again giving the correct 
semantics.

I've verified this cross-version behavior.

While subtle, I think this change is preferable to introducing 
readObject/writeObject methods to do something like explicit write out a 
null value for the element field in the serial form even if element is 
non-null, etc.

The specification updates make the possibility of nulls explicit.

Thanks,

-Joe


--- 
old/src/java.base/share/classes/java/lang/annotation/AnnotationTypeMismatchException.java 
2016-06-30 09:53:08.335033457 -0700
+++ 
new/src/java.base/share/classes/java/lang/annotation/AnnotationTypeMismatchException.java 
2016-06-30 09:53:08.223033453 -0700
@@ -44,7 +44,7 @@
      /**
       * The {@code Method} object for the annotation element.
       */
-    private final Method element;
+    private final transient Method element;

      /**
       * The (erroneous) type of data found in the annotation. This string
@@ -57,10 +57,12 @@
       * Constructs an AnnotationTypeMismatchException for the specified
       * annotation type element and found data type.
       *
-     * @param element the {@code Method} object for the annotation element
+     * @param element the {@code Method} object for the annotation
+     * element, may be {@code null}
       * @param foundType the (erroneous) type of data found in the 
annotation.
       *        This string may, but is not required to, contain the value
-     *        as well.  The exact format of the string is unspecified.
+     *        as well.  The exact format of the string is unspecified,
+     *        may be {@code null}.
       */
      public AnnotationTypeMismatchException(Method element, String 
foundType) {
          super("Incorrectly typed data found for annotation element " + 
element
@@ -71,8 +73,11 @@

      /**
       * Returns the {@code Method} object for the incorrectly typed 
element.
+     * The value may be unavailable if this exception has been
+     * serialized and then read back in.
       *
-     * @return the {@code Method} object for the incorrectly typed element
+     * @return the {@code Method} object for the incorrectly typed
+     * element, or {@code null} if unavailable
       */
      public Method element() {
          return this.element;
@@ -81,7 +86,8 @@
      /**
       * Returns the type of data found in the incorrectly typed element.
       * The returned string may, but is not required to, contain the value
-     * as well.  The exact format of the string is unspecified.
+     * as well.  The exact format of the string is unspecified and the 
string
+     * may be {@code null}.
       *
       * @return the type of data found in the incorrectly typed element
       */